STRATIFICATION 699 



the Latin for bed, strata in the plural) is used for each section of 

 the formation that consists throughout of approximately the same 

 kind of rock material. Thus, if shale, sandstone and limestone suc- 

 ceed one another in thick masses, each is an independent stratum. 

 A stratum may consist of an indefinite number of beds, and a bed 

 of numberless layers. But the distinction of layer and bed is not 

 always obvious." (Dana-Manual, 5th ed.-p/.) 



(e) Schichten nennt man plattenformige Lagen welche "... 

 durch parallele Flachen begranzt werden, bei weiter Ausdehnung 

 in der Regel, nur geringe Dicke besitzen, und das Product successiver 

 Ubereinanderlagerung bilden." (Credner-Elemente, 8th ed.,-i'5.) 



(f) "The material between two planes of stratification forms 

 a stratum or bed, though if the deposit be very thin it is known as 

 a lamina, and the planes are spoken of as planes of lamination (no 

 hard and fast line can be drawn between strata and laminae ; sev- 

 eral of the latter usually occur in the space of an inch)." (Marr- 

 Principles of Stratigraphical Geology- i*/. ) 



From the foregoing definitions it will be seen that there is a 

 considerable diversity of opinion regarding the value of the term 

 stratum. We may gain a clearer concept if we consider it in the 

 light of its origin. Continuous deposition under the same condi- 

 tions will produce a deposit nearly uniform throughout, and of a 

 thickness commensurate with the rate of deposition, the length of 

 time and the coarseness of the material. A sudden change in con- 

 ditions will bring about an abrupt change in the character of the 

 material deposited. It may be coarser, or it may be finer in grain, or 

 it may be of a wholly different composition. Variation in grain, 

 or texture, unless an abrupt change occurs, is indicative of only 

 minor changes in physical condition of the region. But variation in 

 the composition of the material denotes a change of some magni- 

 tude. This being the case, a decided change in the composition of 

 the material ought to be considered a change in strata, while a 

 change in texture, unless it be a great one, should be considered as 

 of minor value, and therefore should constitute a subdivision of the 

 stratum into layers. The great changes in texture which may con- 

 veniently be regarded as of stratum value are those from one to 

 the other of the three primary textural divisions of the clastic rocks, 

 namely, lutaceous, arenaceous and rudaceous. Thus a rudy.te fol- 

 lowing an arenyte may well be considered a. distinct stratum. But 

 the change from a fine arenyte to a coarse one or vice versa is bet- 

 ter regarded as a change in layers. Where deposition is continu- 

 ous, while increase in the force of the currents and increase in the 

 coarseness of the deposit are progressive, an arenyte may gradually 



